1. Field of the Invention
The present invention is in the field of laminated materials, specifically optically clear rigid materials, and more specifically to glass or plastic laminates utilizing a textured adhesive in their formation.
2. Description of the Related Art
Glass can be laminated with various plastic layers to form laminated safety glass which can be used in windshields for automobiles, windows for buildings and elsewhere. Certain types of plastic and glass can be used to form laminated safety glass that functions as bulletproof (or bullet-resistant) glass. The plastic layer in a glass laminate can have dyes or coatings to impart various optical characteristics to the resulting laminated safety glass. Examples of such optical characteristics include tinting, color, partial reflectivity, infrared reflectivity, infrared absorption, ultraviolet reflectivity, and ultraviolet absorption.
While a variety of different materials can be used as a plastic layer in laminated safety glass, polyethylene terephthalate (PET) is known for use as a plastic layer in laminated glass. Commonly, when PET is used as the plastic layer, the PET layer has an adhesive applied on one or both sides of it; the PET/adhesive layer combination is then sandwiched between two layers of glass in a laminate assembly. The adhesive in such arrangements, applied on at least one side of the PET layer, is often in the form of a preformed poly(vinyl butyral) (PVB) sheet; the PET and PVB layers are stacked on top of each other, with the glass layers then placed external to the PET and PVB layers and the whole stack of PET, PVB and glass layers being laminated.
In order to deal with the issue of air removal in various forms of interlayers, such as those which utilize poly(vinyl butyral) (PVB) sheets as one (or more) adhesive layer(s), the PVB sheets are often embossed with a series of repeating troughs or other patterns, such as, but not limited to, those described in United States Patent Application Publication No. 2008/0268204, or are formed to have various random patterns, such as, but not limited to, those described in United States Patent Application Publication No. 2008/0254302, both of which disclosures are incorporated herein by reference in their entirety.
These adhesive sheets, however, are generally relatively thick, on the order of 1 mm (1000 microns). Thick layers of PVB are generally necessary to allow for sufficient texturing to be applied to the PVB sheet to allow for air flow during de-airing processes and so that the material of the PVB sheet can absorb some of the residual air trapped between layers, further decreasing the size and number of air bubbles in the final glass laminate product.
However, one problem with the use of textured PVB sheets as the adhesive is that, although variations in the thickness of the sheet during the lamination process (i.e., variations associated with the surface roughness of the sheet) may help resolve or address the air bubble issue, such variations can add optical distortion to the laminated safety glass because the variations in the PVB sheet can translate into variations appearing in the resultant glass laminate structure. In effect, if the surface roughness of the PVB sheet does not sufficiently “flow” so that air space is removed and the resultant surface, after lamination, is essentially smooth and flat, the surface roughness (and/or differences in thickness) can be visible as optical distortion. An additional problem associated with a laminate comprising a PET layer between two PVB sheets is the formation of another type of optical distortion referred to as waviness or orange peel. When PET is laminated between two layers of PVB, the PET does not “flow” like the PVB does, so it may look wavy or appear to have a surface that looks like applesauce when laminated, even if there is excellent deairing of the laminate. Another problem associated with a laminate comprising a PET layer between two PVB sheets is the formation of wrinkles, particularly with complex curvature laminates such as windshields.
Therefore, it has generally been considered desirable to use a thinner layer of adhesive in order to decrease optical distortion from variations in the thickness of the adhesive and the use of two thicker layers of adhesive (i.e., two PVB sheets). Further, a thinner layer of adhesive allows a plastic layer to more closely follow the contours of curved glass in certain glass laminates and decrease or eliminate folding or wrinkling of the plastic layer or the adhesive layer during the lamination process, as well as a reduction or elimination of the waviness or applesauce effect of the plastic sheet and reduction or elimination of wrinkles PVB sheets, however, often cannot be made much thinner as they cannot hold surface texturing and may be difficult to work with when they are thinner.
For example, U.S. Pat. No. 6,455,141, the entire disclosure of which is herein incorporated by reference, provides for a smooth thin layer of an adhesive, such as PVB, to be applied to one surface of the plastic sheet. This layer is not provided as a preformed PVB sheet, but is provided as deposited material which is laid down directly on the plastic sheet to form a smooth layer already attached to the plastic layer prior to lamination. Such a thin layer allows for improvements in optical issues associated with curvature and for reduced wrinkling, but is less able to absorb residual air trapped between layers of the glass laminate. In other words, previous efforts have fallen short in simultaneously resolving both the optical distortion issues described above along with the air bubble issue.